1. Field of the Invention
The present invention is generally related to a circuit for driving a grounded load.
2. Description of the Related Art
Many electrical applications require a circuit for driving a signal into a grounded load. The output signal from such a circuit can vary in frequency and amplitude, and may contain a broad spectrum of frequencies and amplitudes depending upon the application. Often, the output signal must quickly transition or step from one level or value to another level or value. This transitioning from one value to another may be accomplished by using a fast Digital-to-Analog Converter ("DAC"). For example, a DAC 10 having N input lines 11, as shown in FIG. 1, could be used to provide an output signal at 2.sup.N different values. In response to input signals on lines 11, an output signal from DAC 10 is generated on line 14 to drive load 12 coupled to the ground 13.
However, using a DAC will produce glitches or spikes in the output signal when transitioning between values as illustrated by dotted curve 21 in FIG. 2. The output signal during transitioning can briefly be some other value (for example a higher value 21b or lower value 21a) than either the value at level one or the value at level two. Curve 20 in FIG. 1 illustrates a preferred output signal transitioning from one value to a second value. The glitches, as illustrated by dotted curve 21, are caused by DAC 10's unequal switching delays for digital inputs on lines 11. These glitches may introduce a greater amount of signal into a particular load 12 than is desirable or specified. This increased signal may unduly stress the load and eventually cause a premature failure of the load.
One way of eliminating the glitches in an output signal includes using a circuit 39, as shown in FIG. 3, which provides one output signal to load 34 from one control device at a time. The plurality of output signals from the respective N control devices are effectively summed on line 33 to obtain the maximum required output signal used in driving load 34. Input signals to circuits 39 are generated on lines 30a-Na to control devices 30-N, respectively. Control devices 30-N are controlled on or off by control signals on control lines 30b-Nb. Control signals on lines 30b-Nb are generated such that only one control device is switched on or off at a time. While circuit 39 eliminates glitches, the output capacitance as observed by load 34 is substantially higher than optimal because some of the N control devices may not be turned on when generating a predetermined output signal. This increased output capacitance increases the response time necessary in transitioning from one output value to another output value. A further disadvantage in using circuit 39 is that a predetermined output signal required in driving load 34 may be much less than the theoretical sum of the respective output signals from the N control devices. In other words, more total output signal capacity is available in circuit 39 than is actually required for driving load 34.
Therefore, it is desirable to have a circuit for driving a grounded load which does not generate output signal glitches when transitioning from one output value to another output value. Further, it is desirable to have circuit with reduced output capacitance thereby improving the response time in stepping from one output value to another output value. The circuit should also efficiently generate the required output signal levels and minimize power consumption.